This invention relates to non-linear optical devices, and more particularly to a non-linear optical device provided with organic materials which show a splendid non-linear optical effect.
An optical device according to this invention may be utilized for generating second and third harmonic waves, optically mixing parametric oscillations, optical switches, optical bistable devices, and the like.
A non-linear optical device according to the invention refers to an optical device which makes optical modulations based on the principle of operation of the non-linear optical effect, see Max Schubert and Bernd Wilhelmi: "Nonlinear Optics and Quantum Electronics", John Wiley & Sons (1986).
The non-linear optical effect has been utilized as the basic principle of various optical modulation elements. For example, the second harmonic generation (SHG) as a secondary non-linear optical effect has been widely used to generate a double wave (wavelength: 532 nm) of which wavelength is half short that of the Nd-YAG laser basic wave (wavelength: 1064 nm), for exciting a dye laser, and parametric oscillations are also used to generate near infra-red pulse beams.
Both Pockels effect and Kerr effect known as the electro-optical effect are kinds of non-linear optical effects and they both have the principle of operation of the optical switch as the object.
The non-linear optical device can be used as the elements of the harmonic generation, optical mixing, optical parametric oscillations, optical switches by utilizing secondary and tertiary non-linear polarizations of the electromagnetic fields. Also it has been attracting attention as an optical bistable device which can serve as basic elements of optical computers of which the realization is anticipated in the future.
Conventionally, inorganic materials and semiconductor materials such as lithium niobate (LiNbO.sub.3), potassium dihydrogenphosphate (KH.sub.2 PO.sub.4, generally called as KDP), gallium arsenide have been considered as materials for attaining the non-linear optical device effect. But, in recent years, organic non-linear optical materials having an extremely high optical reaction rate which are excellent in non-linear optical performance (10-1000 times) compared with those of the above-mentioned and are important for optical bistable devices, etc. have been discovered successively, and the non-linear optical devices utilizing the above mentioned optical bistable devices have been actively developed.
Specific examples of the organic non-linear optical materials are urea, 2-methyl-4-nitroaniline (hereafter referred to as MNA) (JP-A-55-50096), N-(4-nitrophenol)-L-prolinol (hereafter referred to as NPP) (JP-A-59-21665) and the like. In particular, MNA or NPP is known to have a non-linear optical constant more than 100 times that of inorganic materials.
However, the above mentioned organic non-linear optical devices have the following problems and are difficult to be put into practical use.
Firstly, although urea is a material having an absorption in the visible region and has phase matching conditions, its non-linear constant is nearly the same as that of the inorganic materials. Therefore, it has little advantages and it is difficult to treat urea due to its water solubility.
On the other hand, each of MNA and NPP has a large non-linear optical constant and an excellent fundamental properties. On the contrary, these materials have some problems in that a large single crystal which is excellent in quality is difficult to be made, they are inferior in stability, resistance, etc., and the cutoff wavelength is longer, that is, the optical absorption takes place within the visible range.
Especially, the problem of length of the cutoff wavelength needs to be improved from the viewpoint of the practical use such as, for example, application to frequency conversion elements of the second harmonic generation (SHG) and the like. This is also the same when optical processing in the visible range as well as the frequency conversion element of the second harmonic generation (SHG) are taken into consideration.
In MNA, the cutoff wavelength is 480 nm and in NPP it is approximately 490 nm. Accordingly, they may be considered to be unstable as the materials for frequency conversion elements of SHG of the currently-used semiconductor laser wavelength: approximately 800 nm).
Especially, heat stability should be improved from the viewpoint of the practical use. Generally, the optical damage threshold value (momentary optical resistance) of the excellent organic non-linear optical materials is much larger than that of the inorganic materials.
However, the resistance of the organic materials when exposed to light for a long time is not so large, because the light energy is converted into heat and the organic materials usually have a poor heat resistance.
Therefore, heat stability becomes a very important feature in considering practical stability and resistance of organic non-linear optical materials. From this point of view, it is desired that organic non-linear optical materials should have a high melting points (more than 200.degree. C.). Prior to the present invention, organic materials having a great non-linear capability and being in conformity with the above mentioned condition have not yet been found. Especially, the heat resistance of the material should be greatly improved when consideration is given to light treatment with high output. The present inventors are the first to find the materials which satisfy all the requirements as an excellent non-linear optical material which have been left to be found in organic non-linear optical materials.